Stereolithographically packaged, in-process semiconductor die

ABSTRACT

A stereolithographic method and apparatus for forming polymeric structures comprising a plurality of overlying layers, each layer formed by polymerizing a thin layer of liquid photopolymer on a prior layer. Crevices formed at the layer interfaces are filled by a stereolithographic method comprising lifting the multilayered structure from the liquid photopolymer, draining excess liquid therefrom, tilting the structure to provide an acute angle of incidence between an incident radiation beam and a side wall of the object, and applying radiation to the crevice to polymerize at least the surface of a quantity of liquid photopolymer therein. The structure may then be subjected to a separate final full cure to fully harden the structure. An exemplary use is the packaging of electronic components and the like.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of application Ser. No. 09/634,239,filed Aug. 8, 2000, U.S. Pat. No. 6,482,576.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to stereolithography and, morespecifically, to the use of stereolithography in forming multilayerstructures with vertical or near-vertical sides, such structuresincluding packages for semiconductor devices and the like. Mostparticularly, the present invention relates to forming multilayerstructures with sides of enhanced smoothness.

2. State of the Art

In the past decade, a manufacturing technique termed“stereolithography”, also known as “layered manufacturing”, has evolvedto a degree where it is employed in many industries.

Essentially, stereolithography as conventionally practiced, involvesutilizing a computer to generate a three-dimensional (3-D) mathematicalsimulation or model of an object to be fabricated, such generationusually being effected with 3-D computer-aided design (CAD) software.The model or simulation is mathematically separated or “sliced” into alarge number of relatively thin, parallel, usually verticallysuperimposed layers, each layer having defined boundaries and otherfeatures associated with the model (and thus the actual object to befabricated) at the level of that layer within the exterior boundaries ofthe object. A complete assembly or stack of all of the layers definesthe entire object, and surface resolution of the object is, in part,dependent upon the thickness of the layers.

The mathematical simulation or model is then employed to generate anactual object by building the object, layer by superimposed layer. Awide variety of approaches to stereolithography by different companieshas resulted in techniques for fabrication of objects from both metallicand nonmetallic materials. Regardless of the material employed tofabricate an object, stereolithographic techniques usually involvedisposition of a layer of unconsolidated or unfixed materialcorresponding to each layer within the object boundaries, followed byselective consolidation or fixation of the material to at least asemisolid state in those areas of a given layer corresponding toportions of the object, the consolidate or fixed material also at thattime being substantially concurrently bonded to a lower layer. Theunconsolidated material employed to build an object may be supplied inparticulate or liquid form, and the material itself may be consolidatedor fixed or a separate binder material may be employed to bond materialparticles to one another and to those of a previously formed layer. Insome instances, thin sheets of material may be superimposed to build anobject, each sheet being fixed to a next lower sheet and unwantedportions of each sheet removed, a stack of such sheets defining thecompleted object. When particulate materials are employed, resolution ofobject surfaces is highly dependent upon particle size, whereas when aliquid is employed, surface resolution is highly dependent upon theminimum surface area of the liquid which can be fixed and the minimumthickness of a layer which can be generated. Of course, in either case,resolution and accuracy of object reproduction from the CAD file is alsodependent upon the ability of the apparatus used to fix the material toprecisely track the mathematical instructions indicating solid areas andboundaries for each layer of material. Toward that end, and dependingupon the layer being fixed, various fixation approaches have beenemployed, including particle bombardment (electron beams), disposing abinder or other fixative (such as by inkjet printing techniques), orirradiation using heat or specific wavelength ranges.

An early application of stereolithography was to enable rapidfabrication of molds and prototypes of objects from CAD files. Thus,either male or female forms on which mold material might be disposedmight be rapidly generated. Prototypes of objects might be built toverify the accuracy of the CAD file defining the object and to detectany design deficiencies and possible fabrication problems before adesign was committed to large-scale production.

In more recent years, stereolithography has been employed to develop andrefine object designs in relatively inexpensive materials and has alsobeen used to fabricate small quantities of objects where the cost ofconventional fabrication techniques is prohibitive, such as in the caseof plastic objects conventionally formed by injection molding. It isalso known to employ stereolithography in the custom fabrication ofproducts generally built in small quantities or where a product designis rendered only once. Finally, it has been appreciated in someindustries that stereolithography provides a capability to fabricateproducts, such as those including closed interior chambers or convolutedpassageways, which cannot be fabricated satisfactorily usingconventional manufacturing techniques.

To the inventors' knowledge, stereolithography has yet to be applied tomass production of articles in volumes of thousands or millions, oremployed to produce, augment or enhance products including other,pre-existing components in large quantities, where minute componentsizes are involved, and where extremely high resolution and a highdegree of reproducibility of results is required. Furthermore,conventional stereolithography apparatus and methods fail to address thedifficulties of precisely locating and orienting a number of preexistingcomponents for stereolithographic application of material theretowithout the use of mechanical alignment techniques or to otherwiseassuring precise, repeatable placement of components.

In the electronics industry, state-of-the-art packaging of semiconductordice is an extremely capital-intensive proposition. In many cases,discrete semiconductor dice carried on, and electrically connected to,leadframes are individually packaged with a filled polymer material in atransfer molding process. A transfer molding apparatus is extremelyexpensive, costing at least hundreds of thousands of dollars in additionto the multi-hundred thousand dollar cost of the actual transfer moldingdies in which strips of leadframes bearing semiconductor dice aredisposed for encapsulation.

Encapsulative packaging of a semiconductor device already mounted on asubstrate by molding and other presently used methods may be verydifficult, time-consuming and costly. In some cases, the device may bepackaged using a so-called “glob-top” material such as a silicone gel,but the package boundaries are imprecisely defined, a dam structure maybe required to contain the slumping gel material, and the seal achievedis generally nonhermetic.

SUMMARY OF THE INVENTION

The present invention includes a method of forming a preciselydimensioned structure from a photopolymer material by astereolithographic process. The structure is formed by creating one ormore layers of at least partially polymerized material adjacent apreformed electronic component or other small component with a highdegree of precision to create a wall adjacent thereto or, optionally, anencapsulative package therefor. For example, a semiconductor die may beprovided with a protective structure in the form of a layer ofdielectric material having a controlled thickness or depth over oradjacent one or more surfaces thereof. As used herein, the term“package” as employed with reference to electrical components includespartial, as well as full, covering or encapsulation of a givensemiconductor die surface with a dielectric material, and specificallyincludes fabrication of a semiconductor die configured in a so-called“chip-scale” package, wherein the package itself, including the die, isof substantially the same dimensions as, or only slightly larger than,the die itself.

The packaging method of the present invention may be applied, by way ofexample and not limitation, to a die mounted to a leadframe (having adie mounting paddle or in a paddle-less leads-over-chip (LOC), or in aleads-under-chip (LUC) configuration), mounted to a carrier substrate ina chip-on-board (COB) or board-on-chip (BOC) arrangement, asemiconductor die in a so-called “flip-chip” configuration, or in otherpackaging designs, as desired.

The present invention employs computer-controlled, 3-D CAD initiated,stereolithographic techniques to apply protective and alignmentstructures to an electronic component such as a semiconductor die. Adielectric layer or layer segments may be formed over or adjacent asingle die or substantially simultaneously over or adjacent a largenumber of dice or die locations on a semiconductor wafer or otherlarge-scale semiconductor substrate, individual dice or groups of dicethen being singulated therefrom. The package may be formed to cover thelateral surfaces as well as the upper and/or lower surfaces of asemiconductor die.

Precise mechanical alignment of singulated semiconductor dice or largersemiconductor substrates having multiple die locations is not requiredto practice the method of the present invention, which includes the useof machine vision to locate dice and features or other componentsthereon or associated therewith (such as leadframes, bond wires, solderbumps, etc.) or features on a larger substrate for alignment andmaterial disposition purposes.

In one embodiment, packaging for electronic components according to theinvention is fabricated using precisely focused coherent electromagneticradiation in the form of an ultraviolet (UV) wavelength laser undercontrol of a computer and responsive to input from a machine visionsystem such as a pattern recognition system to fix or cure a liquidmaterial in the form of a photopolymer.

A multilayer package structure is formed by placing an object in a bathof photopolymer material to a depth forming a thin liquid layer whichwill comprise the lowermost layer of the package structure. A laser beamof coherent radiation is controllably passed over selected portions ofthe thin layer of photopolymer material for partial polymerizationthereof. The object is then lowered to a depth to form a second thinliquid layer of photopolymer material over the at least partiallypolymerized prior layer, followed by laser exposure. A stack of at leastpartially polymerized layers is thus formed, comprising as manyconsecutive, at least partially superimposed layers as are required toachieve the desired structure height.

In the structure fabrication process, small interstitial horizontalcrevices are defined at the joints between adjacent layers of thestructure. Unpolymerized liquid photopolymer material forms a meniscusin each of the crevices. As such uncured material is typically rinsedfrom the structure after it is removed from the bath, a subsequentcomplete cure of the photopolymer of the structure outside of the bathdoes not fill the crevices, but leaves such crevices as unsightly, roughsurface features which reduce the effective wall thickness of thestructure and may also undesirably collect dust, dirt and moisture.

The present invention includes methods and apparatus for substantiallyeliminating these interlayer crevices and smoothing the joints betweenthe structure layers. Following the exemplary formation of a desiredmultilayer package structure about an object such as a semiconductordie, the die with surrounding package structure is removed from thephotopolymer bath and excess liquid photopolymer drained therefrom. Theobject is then tilted by about 5-90 degrees from the horizontal so as toreorient the side walls thereof to face at least partially upwardly, andthe crevices between horizontal at least partially cured photopolymerlayers, each containing a meniscus of unpolymerized liquid photopolymer,are subjected to radiation of an appropriate wavelength to polymerizethe liquid meniscus material and smooth the exterior surfaces of thepackage structure.

It is also contemplated that the present invention has utility withrespect to the formation of stand-alone structures and not merelystructures fabricated in association with preexisting objects, such asthe aforementioned semiconductor dice or other electronic components.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a schematic side elevation of an exemplary stereolithographyapparatus of the invention suitable for use in practicing the method ofthe present invention;

FIG. 1A is an enlarged cross-sectional side view of a portion of thesupport platform of a stereolithographic apparatus of the inventionforming a simple object;

FIG. 1B is an enlarged cross-sectional side view of a portion of thesupport platform of a stereolithographic apparatus of the inventionforming a package on a semiconductor die;

FIG. 2 is a schematic top elevation of a plurality of workpieces in theform of semiconductor dice disposed on a platform of thestereolithographic apparatus of FIG. 1;

FIG. 3 is a schematic side elevation of a plurality of workpieces in theform of semiconductor dice disposed on a platform of thestereolithographic apparatus of FIG. 1 for packaging in accordance withthe present invention;

FIG. 4 is a schematic cross-sectional side elevation of a semiconductordie undergoing a stereolithographic packaging step in a method of thepresent invention;

FIG. 5 is a schematic cross-sectional side elevation of a semiconductordie undergoing a stereolithographic packaging step in a method alternateto the method of the present invention;

FIG. 6 is a schematic cross-sectional side elevation of a semiconductordie packaged by a stereolithographic packaging method alternative to themethod of the present invention;

FIG. 7 is a schematic cross-sectional side elevation of a semiconductordie undergoing stereolithographic packaging by the method and apparatusof the present invention;

FIG. 8 is a schematic cross-sectional side elevation of a semiconductordie in a step of a stereolithographic packaging method in accordancewith the present invention;

FIG. 9 is a schematic cross-sectional side elevation of a semiconductordie packaged by a stereolithographic packaging method in accordance withthe present invention; and

FIG. 10 is a schematic side elevation of an exemplary stereolithographicapparatus modified in accordance with the invention and shown in awall-smoothing operation of the invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 depicts schematically various components and operation of anexemplary stereolithography apparatus 10 modified to create miniaturemultilayer structures with side walls of perceptibly improvedsmoothness. Those of ordinary skill in the art will understand andappreciate that apparatus of other designs and manufacture may bemodified to practice the method of the present invention. The preferredbasic stereolithography apparatus which may be modified in accordancewith the present invention, as well as conventional operation of suchapparatus, are described in great detail in United States Patentsassigned to 3D Systems, Inc. of Valencia, Calif., such patentsincluding, without limitation, U.S. Pat. Nos. 4,575,330; 4,929,402;4,996,010; 4,999,143; 5,015,424; 5,058,988; 5,059,021; 5,096,530;5,104,592; 5,123,734; 5,130,064; 5,133,987; 5,141,680; 5,143,663;5,164,128; 5,174,931; 5,174,943; 5,182,055; 5,182,056; 5,182,715;5,184,307; 5,192,469; 5,192,559; 5,209,878; 5,234,636; 5,236,637;5,238,639; 5,248,456; 5,256,340; 5,258,146; 5,267,013; 5,273,691;5,321,622; 5,344,298; 5,345,391; 5,358,673; 5,447,822; 5,481,470;5,495,328; 5,501,824; 5,554,336; 5,556,590; 5,569,349; 5,569,431;5,571,471; 5,573,722; 5,609,812; 5,609,813; 5,610,824; 5,630,981;5,637,169; 5,651,934; 5,667,820; 5,672,312; 5,676,904; 5,688,464;5,693,144; 5,695,707; 5,711,911; 5,776,409; 5,779,967; 5,814,265;5,840,239; 5,854,748; 5,855,718; and 5,855,836. The disclosure of eachof the foregoing patents is hereby incorporated herein by thisreference. The stereolithographic apparatus may be modified as describedin co-pending U.S. patent application Ser. No. 09/259,142 filed Feb. 26,1999, assigned to the assignee of the present invention and herebyincorporated herein by this reference. This earlier application relatesto the use of a “machine vision” system with suitable programming of thecomputer controlling the stereolithographic process, eliminating theneed for accurate positioning or mechanical alignment of workpieces towhich material is stereolithographically applied, and expands the use tolarge numbers of workpieces which may have differing orientation, size,thickness and surface topography.

While the workpieces employed in the practice of the preferredembodiment of the method of the invention are, by way of example only,semiconductor dice, wafers, partial wafers, other substrates ofsemiconductor material or carrier substrates bearing integrated circuitson dice or other semiconductor structures, the method and apparatus ofthe invention are applicable to fabricating other products includingworkpieces having the aforementioned variations in orientation, size,thickness and surface topography.

With reference again to FIGS. 1, 1A, and 1B, a 3-D CAD drawing of astructure 40 to be fabricated in the form of a data file is placed inthe memory of a computer 12 controlling the operation of apparatus 10,if computer 12 is not a CAD computer in which the original object designis effected. In other words, an object design may be effected in a firstcomputer in an engineering or research facility and the data filestransferred via wide or local area network, tape, disc, CD-ROM orotherwise as known in the art to computer 12 of apparatus 10 for objectfabrication.

The data is preferably formatted in an STL (for StereoLithography) file,STL being a standardized format employed by a majority of manufacturersof stereolithography equipment. Fortunately, the format has been adoptedfor use in many solid-modeling CAD programs, so often translation fromanother internal geometric database format is unnecessary. In an STLfile, the boundary surfaces of a structure 40 are defined as a mesh ofinterconnected triangles.

Apparatus 10 also includes a reservoir 14 (which may comprise aremovable reservoir interchangeable with other reservoirs containingdifferent materials) of liquid material 16 to be employed in fabricatingthe intended structure 40. In the currently preferred embodiment, theliquid material 16 is a photo-curable polymer (hereinafter“photopolymer”) responsive to light in the UV wavelength range. Thesurface level 18 of the liquid material 16 is automatically maintainedat an extremely precise, constant magnitude by devices known in the artresponsive to output of sensors within apparatus 10 and preferably undercontrol of computer 12. U.S. Pat. No. 5,174,931, referenced above andpreviously incorporated herein by reference, discloses one suitablelevel control system.

A support platform or elevator 20 is shown, having an upper surface 30and moved by platform actuator 36. Platform 20 is precisely verticallymovable by actuator 36 via platform controller 32 in fine, repeatableincrements responsive to control of computer 12, and is located formovement 46 downward into and upward out of liquid material 16 inreservoir 14. In addition, under the actuation of actuator 36, platform20 is controllably tiltable by movement 48 to an acute angle 62 with thehorizontal plane (see FIG. 7). Furthermore, in a preferred embodiment,platform 20 is rotatable by movement 68 about a vertical axis 70. Theplatform 20 and/or structures 40 placed on the platform comprise a baseupon which structures 40 are formed by a stereolithographic process inthis invention.

A laser 22 for generating a beam of light 26 in the UV wavelength rangehas associated therewith appropriate optics and galvanometers. The laserbeam 26 is reflected by reflective apparatus 24 to shape and define beam26 into beam 28, which is directed downwardly to the surface 30 ofplatform 20 and traversed in the X-Y plane, that is to say, in ahorizontal plane, in a selected pattern under control of computer 12.Liquid photopolymer material 16 which is exposed to laser beam 28 as itis scanned in an X-Y plane is at least partially cured thereby to atleast a semisolid state.

Data from the STL files resident in the memory 34 of computer 12 ismanipulated to build a structure 40 one layer 50 at a time. Thestructure 40 is constructed on a base which may comprise the platform20, a pre-existing object 44 on the platform 20, or other object.Accordingly, the data mathematically representing structure 40 isdivided into subsets, each subset representing a slice or layer 50 ofstructure 40. This is effected by mathematically sectioning the 3-D CADmodel into a plurality of horizontal layers 50, a “stack” of such layersrepresenting object 40. Each slice or layer 50 may be from about 0.0001inch to about 0.0300 inch thick. The preferred range of layer thicknessis from about 0.002 inch to about 0.020 inch. A slice or layer 50 with arelatively small layer thickness 52 promotes higher resolution byenabling better reproduction of fine vertical surface features ofstructure 40. On the other hand, a structure 40 formed of layers 50having greater thickness 52 will have fewer layers; thus, it isconstructed with fewer scans of the laser beam 28 and the overallproduction rate is typically higher.

In some instances, a base support or supports 42 for a structure 40 orpre-existing object 44 may also be programmed as a separate STL file.The use of such base supports 42 is exemplified in FIGS. 1A and 1B,which are enlarged views of a portion of the platform 20 on which astructure 40 is to be fabricated. In FIG. 1A, a structure 40 is to beconstructed on prior-formed base supports 42. The exemplary structure 40is depicted as formed of 4 layers 50A, 50B, 50C and 50D, each formed bya scan of a laser. In FIG. 1B, a structure in the form of a protectivepolymeric package 40 is to be formed by STL over a pre-existing object44, e.g., a semiconductor die. Base supports 42 are first fabricated onsurface 30 of the platform 20 to support and attach the die to theplatform. Then, the overlying structure 40 is formed by a plurality oflaser scans, each at a higher elevation. Such supports 42 facilitatefabrication of a structure 40 with reference to a perfectly horizontalplane above the surface 30 of platform 20. The structure 40 may beconstructed upon or adjacent to a pre-existing object 44 such as asemiconductor die, electronic substrate, or the like. The formation of abase support 42 between the pre-existing object 44 and the platformsurface 30 enables rigid and precise positioning of the pre-existingobject 44 in a desired precise orientation on the platform surface.

Where a “recoater” blade 38 is employed as described below, theinterposition of base supports 42 precludes inadvertent contact of blade38 with platform surface 30.

Before fabrication of structure 40 is initiated with apparatus 10, theprimary STL file for structure 40 and the file for base support(s) 42(if used) are merged. It should be recognized that, while reference hasbeen made to a single structure 40, multiple structures 40 may beconcurrently fabricated on surface 30 of platform 20. In such aninstance, the STL files for the various structures 40 and supports 42,if any, are merged. Operational parameters for apparatus 10 are thenset, for example, to adjust the size (diameter, if circular) of thelaser light beam 28 used to cure material 16.

Before initiation of a first support layer for a support 42, or a firstlayer 50A for a structure 40 is commenced, computer 12 automaticallychecks and, if necessary, adjusts by means known in the art asreferenced above, the surface level 18 of liquid material 16 inreservoir 14 to maintain same at an appropriate focal length for laserbeam 28. U.S. Pat. No. 5,174,931, referenced above and previouslyincorporated by reference, discloses one suitable level control system.Alternatively, the height of reflective apparatus 24 may be adjustedresponsive to a detected surface level 18 to cause the focal point oflaser beam 28 to be located precisely at the surface of liquid material16 at surface level 18 if level 18 is permitted to vary, although thisapproach is somewhat more complex.

The platform 20 may then be submerged in liquid material 16 in reservoir14 to a depth equal to the thickness 52 of one layer or slice 50 of thestructure 40, and the liquid surface level 18 readjusted as required toaccommodate liquid material 16 displaced by submergence of platform 20.Laser 22 is then activated so that laser beam 28 will scan liquidmaterial 16 over surface 30 of platform 20 to at least partially cure(e.g., at least partially polymerize) liquid material 16 at selectedlocations, defining the boundaries of a first layer 50 (of structure 40or support 42, as the case may be) and filling in solid portionsthereof.

Platform 20 is then lowered by a distance equal to the thickness 52 of alayer 50, and the laser beam 28 scanned to define and fill in the secondlayer 50B while simultaneously bonding the second layer to the first.The process is then repeated, layer by layer, until structure 40 iscompleted.

If a recoater blade 38 is employed, the process sequence is somewhatdifferent. In this instance, the surface 30 of platform 20 is loweredinto liquid material 16 below surface level 18, then raised thereaboveuntil it is precisely one layer's thickness 52 below blade 38. Blade 38then sweeps horizontally over surface 30, or (to save time) at leastover a portion thereof on which a structure 40 is to be fabricated, toremove excess liquid material 16 and leave a film thereof of theprecise, desired thickness 52 on surface 30. Platform 20 is then loweredso that the surface of the film and material level 18 are coplanar andthe surface of the material 16 is still. Laser 22 is then initiated toscan with laser beam 28 and define the first layer 50A. The process isrepeated, layer by layer, to define each succeeding layer 50 andsimultaneously bond same to the next lower layer 50 until structure 40is completed. A more detailed discussion of this sequence and apparatusfor performing same is disclosed in U.S. Pat. No. 5,174,931, previouslyincorporated herein by reference.

As an alternative to the above approach to preparing a layer of liquidmaterial 16 for scanning with laser beam 28, a layer of liquid material16 may be formed on surface 30 by lowering platform 20 to flood material16 over surface 30 or over the highest completed layer 50 of structure40, then raising platform 20 and horizontally traversing a so-called“meniscus” blade across the platform 20 (or just the formed portion ofstructure 40) one layer thickness 52 thereabove, followed by initiationof laser 22 and scanning of beam 28 to define the next higher layer 50.

Yet another alternative to layer preparation of liquid material 16 is tomerely lower platform 20 to a depth equal to that of a layer of liquidmaterial 16 to be scanned, and then traverse a combination flood bar andmeniscus bar assembly (not shown) horizontally over platform 20 (ormerely over structure 40) to substantially concurrently flood liquidmaterial 16 over platform 20 and define a precise layer thickness 52 ofliquid material 16 for scanning.

All of the foregoing methods and apparatus for liquid material floodingand layer thickness control are known in the art.

Each layer 50 of structure 40 is preferably built by first defining anyinternal and external object boundaries of that layer 50 with laser beam28, then hatching solid areas of object 40 with laser beam 28. If aparticular part of a particular layer 50 is to form a boundary of a voidin the structure 40 above or below that layer 50, then the laser beam 28is scanned in a series of closely-spaced, parallel vectors so as todevelop a continuous surface or skin with improved strength andresolution. The time it takes to form each layer 50 depends upon itsgeometry, surface tension and viscosity of material 16, thickness 52 ofthe layer, and laser scanning speed.

In practicing the present invention, the stereolithography apparatus 10preferably comprises a commercially available STL system which ismodified by the invention to enable smoothing of vertical sides 54 ofSTL-formed structures 40. For example and not by way of limitation, theSLA-250/50HR, SLA-5000 and SLA-7000 stereolithography systems, eachoffered by 3D Systems, Inc, of Valencia, Calif., are suitable formodification. Liquid photopolymers 16 believed to be suitable for use inpracticing the present invention include Cibatool SL 5170 and SL 5210resins for the SLA-250/50HR system, Cibatool SL 5530 resin for theSLA-5000 system, and Cibatool SL 7510 resin for the 7000 system. All ofthese resins are available from Ciba Specialty Chemicals Corporation. Byway of example and not limitation, the layer thickness of material 16 tobe formed, for purposes of the invention, may be on the order of about0.0001 to about 0.030 inch, and more preferably, from about 0.001 toabout 0.020 inch, with a high degree of uniformity over a field on asurface 30 of a platform 20. It should be noted that layers 50 havingdiffering thicknesses 52 may be used to construct a structure 40, so asto form a structure 40 of a precise, intended total height 72 or toprovide different material thicknesses 52 for different portions of thestructure 40.

The size of the laser beam “spot” 74 impinging on the surface of liquidmaterial 16 to cure same may generally be on the order of 0.002 inch to0.008 inch, using presently available STL equipment. Resolution ispreferably about ±0.0003 inch in the X-Y plane (parallel to surface 30)over at least a 0.5 inch×0.25 inch field from a center point, permittinga high resolution scan effectively across a 1.0 inch×0.5 inch area. Ofcourse, it is desirable to have substantially this high a resolutionacross the entirety of surface 30 of platform 20 to be scanned by laserbeam 28. This area may be termed the “field of exposure”, such areabeing substantially coextensive with the vision field of a machinevision system employed in the apparatus of the invention as explained inmore detail below. The longer and more effectively vertical the path oflaser beam 26, 28, the greater the achievable resolution.

Referring again to FIG. 1 of the drawings, it should be noted thatapparatus 10 of the present invention includes a camera 76 which is incommunication with computer 12 and preferably located, as shown, inclose proximity to optics and scan controller, i.e., reflectiveapparatus 24 located above surface 30 of platform 20. Camera 76 may beany one of a number of commercially available cameras, such ascapacitive-coupled discharge (CCD) cameras available from a number ofvendors. Suitable circuitry as required for adapting the output ofcamera 76 for use by computer 12 may be incorporated in a board 82installed in computer 12, which is programmed as known in the art torespond to images generated by camera 76 and processed by board 82.Camera 76 and board 82 may together comprise a so-called “machine visionsystem”, and specifically, a “pattern recognition system” (PRS),operation of which will be described briefly below for a betterunderstanding of the present invention. Alternately, a self-containedmachine vision system available from a commercial vendor of suchequipment may be employed. For example, and without limitation, suchsystems are available from Cognex Corporation of Natick, Mass. Forexample, the apparatus of the Cognex BGA Inspection Package™ or the SMDPlacement Guidance Package™ may be adapted to the present invention,although it is believed that the MVS-8000™ product family and theCheckpoint® product line, the latter employed in combination with CognexPatMax™ software, may be especially suitable for use in the presentinvention.

It is noted that a variety of machine vision systems are in existence,examples of which and their structures and uses are described, withoutlimitation, in U.S. Pat. Nos. 4,526,646; 4,543,659; 4,736,437;4,899,921; 5,059,559; 5,113,565; 5,145,099; 5,238,174; 5,463,227;5,288,698; 5,471,310; 5,506,684; 5,516,023; 5,516,026; and 5,644,245.The disclosure of each of the immediately foregoing references is herebyincorporated by this reference.

In order to facilitate practice of the present invention with apparatus10, a data file representative of the size, configuration, thickness andsurface topography of a pre-existing object 44, for example, aparticular type and design of semiconductor die to be packaged, isplaced in the memory 34 of computer 12. If the pre-existing object 44,i.e., die, is to be packaged with a leadframe, data representative ofthe die with attached and electrically connected leadframe is provided.If packaging material in the form of the aforementioned photopolymermaterial 16 is to be applied only to an upper surface 86 (or portionsthereof excluding active surface structures 92) of a die 44 to formupper package surface 88, or to the upper surface 86 and portions or allof the side surfaces 84 of a die, a large plurality of such dice 44 maybe placed on surface 30 of platform 20 for packaging, as depicted inFIGS. 2 and 3. If package sides 54 are to be formed, it is desirablethat the surface 30 of platform 20 comprise, or be coated or coveredwith, a material from which the at least partially cured material 16defining the lowermost layers of the package side wall 54 may be easilyreleased to prevent damage to the packaging. Alternatively, a solventmay be employed to release the package side walls 54 from platform 20after packaging is completed. Such release and solvent materials areknown in the art. See, for example, U.S. Pat. No. 5,447,822 referencedabove and previously incorporated herein by reference.

Following mounting of the dice 44 on platform 20, camera 76 is thenactivated to locate the position and orientation of each die 44 to bepackaged by scanning platform 20 and comparing the features of the dice44 with those in the data file residing in memory 34, the locational andorientational data for each die 44 then also being stored in memory. Itshould be noted that the data file representing the design size, shapeand topography for the dice 44 may be used at this juncture to detectphysically defective or damaged dice 44 prior to packaging and toautomatically delete such dice 44 from the packaging operation. Itshould also be noted that data files for more than one type (size,thickness, configuration, surface topography) of die 44 may be placed incomputer memory 34 and the computer 12 programmed to recognize not onlydie locations and orientations, but which type of die 44 is at eachlocation so that material 16 may be cured by laser beam 28 in thecorrect pattern and to the height required to define package side walls54 and to provide a package top surface 88 at the correct level and ofthe correct size and shape over each die 44.

Continuing with reference to FIGS. 1, 1A and 1B of the drawings, dice 44on platform 20 may then be partially submerged below the surface level18 of liquid material 16 to a depth the same as, or greater than, thethickness of a first layer of material 16 to be at least partially curedto a semisolid state to form the lowest layer 50A of a package side wall54 about each of dice 44, and then raised to a depth equal to the layerthickness, the surface of liquid material 16 being allowed to settle.The material 16 selected for use in packaging dice 44 may be one of theabove-referenced resins from Ciba Specialty Chemical Company whichexhibits a desirable dielectric constant, is of sufficient(semiconductor grade) purity, and which is of sufficiently similarcoefficient of thermal expansion (CTE) so that the package structure,i.e., structure 40 and the die 44, itself is not stressed during thermalcycling in testing and subsequent normal operation.

Laser 22 is then activated and scanned to direct beam 28, under controlof computer 12, about the periphery of each die 44 to effect theaforementioned partial cure of material 16 to form a first layer 50A.The platform 20 is then lowered into reservoir 14 and raised to anotherside wall layer thickness-equaling depth increment 52 and the laser 22activated to add another side wall layer 50B. This sequence continues,layer 50 by layer 50, until the package side walls 54 are built up aboutdice 44. A final layer or layers 50 may be applied over a portion or theentirety of the upper surface 86 of dice 44, forming an upper packagesurface 88 thereon. The layer thicknesses 52 may be controlled todiffer, depending upon the thickness required for the top of thepackage. For example, a greater total thickness of material 16 may berequired to cover a die 44 having wire bonds protruding upwardlytherefrom than if a die 44 is covered before connection to a leadframe.It should also be noted that the total thickness of material 16 over aselected portion of a given die 44 may be altered die by die, againresponsive to output of camera 76 or one or more additional cameras 78or 80, shown in broken lines, detecting the protrusion of unusually highwire bond loops or other features projecting above the active surface ofa given die 44 which should be, but is not, covered by the “design” orpreprogrammed thickness of material 16 disposed over and at leastpartially cured on upper die surface 86. In any case, laser 22 is againactivated to at least partially cure material 16 residing over each die44 to form a package top 94 of one or more layers 50, top 94 beingsubstantially contiguous with package side walls 54. Laser beam 28 iscontrolled as desired to avoid certain surface features on dice 44, suchas bond pads, which are intended to be exposed for connection tohigher-level packaging by wire bonding, tape automated bonding (TAB)using flex circuits, or the use of projecting conductive connectors suchas solder bumps in a “flip-chip” configuration. It should also be notedthat the package top 94 may be formed within an outer boundary definedby side walls 54 extending above upper (active) surface 86 and forming adam thereabout. In this instance, the platform 20 may be submerged sothat material 16 enters the area within the dam, raised above surfacelevel 18, and then laser beam 28 activated and scanned to at leastpartially cure material 16 residing within the dam. Alternatively, a“skin” may be cured by STL over the top surface 86 of the die 44, andliquid polymer 16 entrapped thereby will be subsequently cured in afinal curing step.

When the final layer 50 _(n) is formed to complete a selected portion ofthe structure 40, platform 20 is elevated above surface level 18 ofliquid material 16 and excess liquid 16 is drained from the STL-formedstructure 40. At this stage, depicted in FIG. 4, the surfaces 66 ofvertical sides 54 of the at least partially polymerized structure 40 maybe somewhat nonplanar, having linear, slit-like, external horizontalcrevices 56 at the interfaces 58 between adjacent layers 50A, 50B, 50Cand 50D, as well as between layer 50A and platform 20. A meniscus 60comprising a quantity of unpolymerized liquid material 16 is retained ortrapped within each crevice 56. Where the vertical sides 54 meet the die44, similar interior crevices 96 may occur along the layer interfaces58, being filled with unpolymerized photopolymer material 16 which istrapped therein.

Where the initial lack of planarity of the surfaces 66 of the verticalsides 54 may be tolerated, the structure 40 may be washed to remove allunpolymerized material 16 from the external surfaces 30, 66 and 88,including the external crevices 56. The washed structure 40 is shown inFIG. 5, being free of liquid photopolymer 16 in the external crevices56. It should be noted that at this stage, the polymer comprisingstructure 40 is typically in various stages of polymerization, includingliquid polymer 16 trapped in internal crevices 96. Furthermore, theuppermost layer 50D may comprise a polymerized “skin” which trapsunpolymerized or partially polymerized material 16 therebelow. Followingremoval of the structure 40 from the platform 20, a final curing steppolymerizes and consolidates the structure 40, including any liquidpolymer 16 in the internal crevices 96 or otherwise trapped within thestructure. As exemplified in FIG. 6, empty external crevices 56 remainin the side walls 54 of the package, i.e., structure 40, followingremoval from the platform 20 and full cure of the structure 40. Thesecrevices 56 reduce the effective side wall thickness 102 (see FIG. 6)and represent potential weaknesses in the packaging 44. Dust, otherdebris and moisture may collect in the crevices 56.

The method and apparatus of the present invention pertain to thestereolithographic polymerization of liquid polymer 16 retained inexternal crevices 56 shadowed by overlying polymerized layers 50 of astructure 40 formed by STL, whereby the side wall surfaces 66 are madesmooth. The smoothing method is enabled by certain modifications toconventional STL apparatus, described infra. The smoothing fills thecrevices 56 to a depth approaching the flat side wall surfaces 66, thusremoving locations where dust and moisture may collect. Smoothing alsoincreases the effective thickness of the protective layer over the die44 for uniform protection, increases resistance of the formed structure40 to damage and environmental contamination by providing a strongerstructure, reduces waste of polymeric material, and is aestheticallypreferred.

In accordance with the present invention, removal of the platform 20with structure 40 from the liquid photopolymer material 16 is followedby draining of excess material 16 therefrom, resulting in theconfiguration depicted in FIG. 4. Residual liquid photopolymer material16 retained in the external crevices 56 and having outer meniscussurfaces 60 is not removed.

As shown in FIG. 7, the platform 20 on which the object 40 is formed isthen reoriented or tilted about a horizontal axis 104 to an acute angle62 from the horizon. Crevice 56 with liquid meniscus 60 in a verticalside 54 is then irradiated by scanning of the laser beam 28 at incidenceangle 64 to polymerize the photopolymer. At a minimum, a thin “skin” 98of partially polymerized material must be formed to contain anyadditional unpolymerized liquid material 16 during subsequent washing(see FIG. 8). Polymerization of the meniscus liquid 16 results in asmooth surface 66 of the vertical side 54. The incident laser beam 28 isgenerally in a vertical orientation, whereby the angle of incidence 64between beam 28 and the side surface 66 is equal to angle 62, and may beany acute angle between about 5 degrees and about 90 degrees. Thepreferred angle of incidence 64 is between about 10 degrees and about 60degrees. The desirable angle of incidence 64 to achieve a smooth, fullypolymerized surface 66 depends upon the depth of the crevice 56, theconcavity or convexity of the liquid meniscus 60, and the degree towhich the liquid meniscus is shadowed by the overlying layer 50.

As shown in FIG. 8, the side surfaces 66 of structure 40 are smoothed bythe tilted STL formation of “skins” 98 of polymerized photopolymer,which typically entrap unpolymerized material 16 in internal pockets 100in the side walls 54. The formation of a surface skin 98 avoids the useof a high incidence angle 64 such as>60 degrees to reach the innermostportions of the crevices 56 and is achieved with minimum laser energy.

Following the STL smoothing step, any excess uncured liquid material 16residing on the surfaces of structure 40 may be manually removed andstructure 40 may then be solvent-cleaned and removed from platform 20,usually by cutting it free of base supports (not shown). Structure 40will then be generally subjected to postcuring, as material 16 istypically only partially polymerized and exhibits only a portion(typically 40% to 60%) of its fully cured strength. Postcuring toenhance and accelerate consolidation and complete hardening of structure40 may be effected in another apparatus projecting broad-source UVradiation in a continuous manner over structure 40, and/or by thermalcompletion of the initial, UV-initiated partial cure, and/or by othercuring means.

In this manner, a structure, i.e., package 40 depicted in FIG. 9, may beformed with smooth, uniform thickness side wall surfaces 66 in minimaltime within apparatus 10 and, optionally, a final cure apparatus such asis well known in the art. In instances where a plurality of structures40 are formed on a relatively large platform 20, it is desirable thatplatform actuator 36 have the capability of horizontally translatingplatform 20 above the top of reservoir 14 and while in a tilted positionto place tilted walls of each structure 40 directly below laser beam 28.

In reference to FIGS. 1 and 10, a preferred embodiment of the apparatus10 includes a platform 20 which is precisely movable in a verticaldirection, i.e., along vertical axis Z, and may be tilted verticallyabout a horizontal axis or axes 104 (see FIG. 7). In a further preferredembodiment, platform 20 is also rotatable about an axis 70 normal to theplatform surface 30. Thus, once the platform 20 is tilted to a desiredincidence angle, it may then be rotated about axis 70 to present eachside wall 54 in turn to the substantially vertical laser beam 28. Thelaser beam 28 may be scanned over each longitudinal crevice 56 of aselected side wall 54 before the platform 20 rotates for presentation ofthe next side wall 54. Thus, the number of tilting operations andscanning steps may be minimized. Alternately, the platform 20 may beconfigured to be first rotated about an axis 70 to a desired positionand then tilted about horizontal axis 104. The platform may then berotated further about axis 70 as desired to present additional sidewalls 54 to laser beam 28. Alternatively, the platform 20 may be rightedto a horizontal position after each side wall 54 facing in a particulardirection is exposed to laser beam 28, rotated about axis 70 and thenre-tilted.

It should be recognized that where a plurality of structures 40 areformed with varying configurations on a platform 20, an initial tiltingstep followed by platform rotation and X-Y laser scanning of each objectis readily accomplished. The (a) tilting, rotation and Z axis movementof the platform 20 and (b) the scanning operations of the laser beam 28are both controlled by a computer program using the data file alreadypresent in the computer memory 34. As previously described, such datamay include, for example, at least one parameter such as the size,configuration, thickness and surface topography of each device to bepackaged, together with the construction details of the package to beformed.

The STL smoothing step may be performed by merely lifting the platform20 above the photopolymer reservoir 14, tilting and scanning withinapparatus 10. Incident or reflected laser radiation from the smoothingstep may be directed undesirably downwardly into the reservoir 14. Thus,as shown in FIG. 1, an opaque member 106 is provided which is movableacross the reservoir 14 to shield liquid photopolymer 16 from reflectedor incident laser radiation during the smoothing step. The opaque member106 must be opaque to laser radiation and be resistant to damagetherefrom. The opaque member 106 may comprise a somewhat flexible rollof material or a rigid plate which slides over the reservoir 14, forexample. Where the smoothing step is not performed adjacent thereservoir 14, use of the opaque member 106 is not required. Such is thecase if the smoothing step is performed by a separate laser apparatus orthe reservoir 14 is removed from STL apparatus 10 prior to the smoothingstep.

While the smoothing step has been described above in the context ofusing a laser beam 28 scanned along crevices 56, it will be appreciatedthat a broad beam or flood type radiation source of appropriatewavelength or wavelength range may be used to expose liquid photopolymermaterial 16 in all crevices 56 facing in a given direction at the sametime. Moreover, simultaneous exposure of all side walls 54 for smoothingmight be accomplished either within or outside apparatus 10 through theuse of a plurality of broad beam or flood type radiation sourcessurrounding and above platform 20 or other supporting platform andfacing downwardly at an appropriate angle. Of course, the severity ofthe angle required for orientation of the radiation sources would dependon the spacing between adjacent structures 40 on platform 20.

While the foregoing example of the invention shows the packaging of adie 44 on the top 86 and sides 84, the die bottom 108 (see FIG. 9) mayalso be stereolithographically packaged in a variety of configurationsto effect substantially complete sealing of the die. For example, thedice 44 may be placed bottom 108 down on an insulative material whichwill comprise the bottom packaging and STL used to construct the sidewalls 54 using the insulative material as a base. Alternatively, apackage bottom is first formed on the platform 20 by stereolithography,the die 44 is placed on the package bottom, and the STL process iscontinued to erect the side walls 54. In yet another variation, where aplurality of dice 44 are secured and electrically connected (as by wirebonding, thermocompression bonding, TAB bonding or otherwise as known inthe art) to leadframes, the leadframe may be inverted to form the bottompackaging by STL. In still another variation, dice 44 may beencapsulated on five sides and then inverted and encapsulated on thesixth, whether it be a “top”, bottom” or “side”. Using machine visionsystems, mere inversion of the dice 44 after all but one side of each iscovered and reinitiation of laser scanning may be used to complete thepackages. If certain die features such as bond pads, solder bumps, etc.,are to remain unencapsulated, the apparatus 10 may be programmed torecognize and avoid such features.

While the prior discussion describes the invention in terms of thepackaging of the top 86 and four lateral sides 54 of a semiconductor die44, the method of the present invention is more broadly applicable tothe formation of any structure 40 which is formed from a photopolymer 16in layers 50 by stereolithography. Thus, the structure 40 may standalone or be attached to or adjacent to another object such as a die 44.The small size of semiconductor devices makes the use ofstereolithography particularly advantageous for forming protectivepackaging and other structures 40 on devices and electronic substrates.

It is notable that the method of the present invention produces asubstantially smooth side wall surface 66 without consuming anyadditional photopolymer in comparison to not using the inventive method,and in fact, may enable (due to enhanced uniformity of wall thickness) adie package wall to be formed with a reduced thickness 102, furtherreducing the already small quantity of polymer material 16 consumed instereolithographic packaging. In addition, the capital equipment expenseof transfer molding processes is eliminated and the inventive method isextremely frugal in its use of dielectric encapsulant material 16, sinceall such material in which cure is not initiated by laser 22 remains ina liquid state in reservoir 14 for use in packaging the next pluralityof dice 44 or other objects. Further, since it is no longer necessary toencapsulate dice 44 with packaging of sufficient wall thickness toaccommodate relatively large dimensional variations such as those whichmay be exhibited by wire bond loop heights, the overall volume ofpackaging material may be smaller in some cases. Also, the packagedimensional tolerances achievable through use of the present inventionare increased in precision in comparison to transfer molded packaging.Moreover, there is no potential for mold damage, mold wear, orrequirement for mold refurbishment. Finally, the extended cure times atelevated temperatures for transfer molded packaging, on the order of,for example, four hours at 175° C., required after removal of batches ofdice from the transfer mold cavities, are eliminated. Post-cure of diepackages formed according to the present invention may be effected withbroad-source UV radiation emanating from, for example, flood lights in achamber through which dice are moved on a conveyor or in large batches.Curing in an oven at, for example, 160° C., is another option whicheffects full curing of liquid polymer 16 in interior crevices 96 andinternal pockets 100.

Full curing of unpolymerized material 16 retained in the crevices 56without a prior skin formation is not practical because, without a priorsolvent wash, droplets and films of liquid material persist on thesurfaces of the structure 40. As a result, full curing without a priorsolvent wash results in substantially nonuniform side wall surfaces 66and upper surface 88. If the structure 40 is first solvent-washedwithout “skinning” of the meniscus photopolymer material 16 in crevices56, the liquid polymer 16 is washed from the crevices 56 so the crevicesremain after full cure.

It should also be noted that the packaging method of the presentinvention is conducted at substantially ambient temperature, the smallbeam spot size and rapid traverse of laser beam 28 around and over thestructures 44 resulting in negligible thermal stress thereon. Physicalstress on structures 44, i.e., the semiconductor dice and associatedleadframes and bare wires, is also significantly reduced, in thatmaterial 16 is fixed in place and not moved over the dice in a viscous,high-pressure wave front as in transfer molding, followed bycooling-induced stressing of the package. Bond wire sweep is eliminated,as is any tendency to drive particulates in the polymer encapsulantbetween lead fingers and an underlying portion of the active surface ofthe die with consequent damage to the integrity of the active surface.

While the present invention has been disclosed in terms of certainpreferred embodiments, those of ordinary skill in the art will recognizeand appreciate that the invention is not so limited. Additions,deletions and modifications to the disclosed embodiments may be effectedwithout departing from the scope of the invention as claimed herein.Similarly, features from one embodiment may be combined with those ofanother while remaining within the scope of the invention.

What is claimed is:
 1. A packaged, in-process semiconductor die,comprising: a semiconductor die; one or more walls surrounding thesemiconductor die and formed of superimposed layers of at leastsemisolid polymeric material, the superimposed layers defining crevicestherebetween on exterior surfaces of the one or more walls; outer skinsof at least semisolid polymeric material extending over at least some ofthe crevices on the one or more exterior surfaces of the one or morewalls; and liquid polymeric material within the crevices behind theouter skins.
 2. The packaged, in-process semiconductor die of claim 1,wherein the liquid polymeric material comprises a photopolymer.
 3. Thepackaged, in-process semiconductor die of claim 1, wherein the outerskins comprise partially polymerized photopolymer skins.
 4. Thepackaged, in-process semiconductor die of claim 1, wherein each of theone or more walls has a thickness of between about 0.0001 inch and0.0300 inch.